Activation of - N=CH - bond in a Schiff base by divalent nickel monitored by NMR evidence

Article abstract of DOI:10.1002/mrc.2857

The Schiff base, 2-salicylidene-4-aminophenyl benzimidazole in ethanol undergoes activation of -N=CH- bond by Ni²⁺ in the presence of ammonia or primary alkyl amine to produce nickel complexes of the formula Ni{o-C₆H₄(O)CH NR}₂. n H₂O [R=H, Me; n=0; R=Et, n=0.5] and 4-aminophenyl benzimidazole. The products have been identified by elemental analysis, magnetic susceptibility measurements and IR, ESR, mass and extensive NMR spectral studies. The possible mechanism for the activation of -N=CH - bond has also been proposed. Copyright

Full text of DOI:10.1002/mrc.2857

Research Article
Received: 27 July 2011
Revised: 24 October 2011
Accepted: 25 October 2011
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/mrc.2857
Activation of – N=CH – bond in a Schiff base by
divalent nickel monitored by NMR evidence
a,b
a
a
M. Chandrakala, N. M. Nanje Gowda, K. G. S. Murthy and
b
K. R. Nagasundara *
2
+
The Schiff base, 2–salicylidene–4–aminophenyl benzimidazole in ethanol undergoes activation of –N=CH– bond by Ni in the
presence of ammonia or primary alkyl amine to produce nickel complexes of the formula Ni{o–C (O)CH NR} . n H O [R = H,
6
H
4
2
2
Me; n = 0; R = Et, n = 0.5] and 4–aminophenyl benzimidazole. The products have been identified by elemental analysis,
magnetic susceptibility measurements and IR, ESR, mass and extensive NMR spectral studies. The possible mechanism for
the activation of –N=CH – bond has also been proposed. Copyright © 2012 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
1
Keywords: Ni (II)-Schiff base complexes; 2–salicylidene–4–aminophenyl benzimidazole; NMR; H 4Q-SQ correlation spectroscopy; ESR;
magnetic susceptibility
2
+
undergoes cleavage assisted by Ni ions in the presence of
ammonia or a primary alkyl amine, and the Schiff base is
reformed without the phenyl benzimidazole part. The nitrogen of
the reformed Schiff base is bound to the metal ion.
Introduction
Divalent nickel is known to aid template syntheses involving
the condensation reaction of an imine (Schiff base) and a ketone
[
1]
2+
to produce a macrocycle. Furthermore, Ni ions react with
N-alkyl salicylaldimines to produce four-coordinate complexes
bis(N-alkylsalicylaldiminato)nickel(II), which exhibit configura-
[
2]
Experimental
tional isomerism. A majority of these complexes are square
planar (diamagnetic), and the remaining are tetrahedral (paramag-
Instrumentation
[3]
netic); the equilibrium between the two being influenced by the
steric factor of the substituents on the Schiff base. Solid state
magnetic moments (meff) would be around 0.2 BM for square planar
The conductivity measurements were made on a digital conduc-
tivity meter (Elico model – 180) at room temperature. Elemental
analyses were carried out using Elementer Vario EI 111 and Carlo
Erba–1108 instruments.
[
4]
and 3.2 BM for tetrahedral Ni(II) environments. An equilibrium
mixture has a magnetic moment of around 1.6 BM. In some of
the complexes, the diamagnetic character in the solid state turns
into paramagnetic in the molten state or in organic solvent
The IR spectra of the complexes (KBr Pellet) were recorded on a
-1
Nicolet Impact–400D spectrometer in the range 4000–400 cm .
Mass spectra of 4-aminophenyl benzimidazole and the nickel
complexes were obtained on an ESI, esquine 300 plus, Bruker
Daltonics and JEOL JMS GC-MS instruments.
Room temperature magnetic moment measurements were
carried out using the Guoy method using Hg[Co(SCN) ] as the calibrant.
4
Electronic spectra of the complexes were recorded in the range
200–1100 nm on a Elico SL 159 UV–visible spectrophotometer in
DMF/chloroform solvents.
[5]
solutions. Sacconi and coworkers have observed that diamagnetic
Ni(N-Me.sal) turns into paramagnetic with two unpaired electrons
on heating to above 150 C. The work of Hoss and Elias has
revealed that square planar Ni(N–alkyl Sal) complexes are
co-ordinatively unsaturated and add on nitrogen donor ligands to
2
ꢀ
[5]
2
[6]
produce six-coordinate complexes. Metal complexes containing
Schiff base ligands have gained significance in view of varied range
[7–13]
of applications,
structural chemistry, catalysis, chemical and
photochemical reactions, magnetism, gas storage, chemically
modified electrodes, etc. Imines play important roles in biological
processes. They are crucial in the metabolism of carbohydrates
and amino acids. They also provide structural support within
biomolecules. Activation of –N=CH– groups followed by cleavage
and reformation of a new imine bond in the presence of a primary
amine has been visualized in biological (in vivo) systems as
ESR spectra were recorded using a Bruker EMX X-band Instru-
ment. The metal content was determined using Atomic Absorption
spectrometer ECIL model–4139.
*
Correspondence to: K. R. Nagasundara, Department of Chemistry, Central
College Campus, Bangalore University, Bangalore-560 001, India. E-mail:
kudernag@yahoo.com
[
14]
illustrated in Scheme 1.
Thus, the study of the activation of
–N=CH– is of considerable importance, and we have noticed such
a Postgraduate Department of Chemistry, V. V. Pura College of Science,
Bangalore, India
a phenomenon in in vitro chemical reactions.
In the present investigation, it has been observed that the
acyclic nitrogen-carbon double bond of the Schiff base (I)
b Department of Chemistry, Central College Campus, Bangalore University,
2–salicylidene–4–aminophenyl benzimidazole (SAPbzlH) in ethanol
Bangalore, India
Magn. Reson. Chem. 2012, 50, 335–340
Copyright © 2012 John Wiley & Sons, Ltd.

M. Chandrakala et al.
was 16, and a relaxation delay of 1.5 s was used between transi-
ents. Gradient ratio was 50 : 30 : 40. For HETCOR, spectral widths
of 14 and 220 ppm were employed in F
2 1
and F dimensions,
respectively, and delay responsible for maximum transfer of
13
1
magnetization was optimized for C– H coupling of 145 Hz.
The data size for the acquisition was 128 ꢁ 4096 with eight scans
for each increment and a relaxation delay of 5 s between the free
induction decays. Gradient ratio employed was 80 : 20. For
DEPT-45, DEPT-90 and DEPT-135 experiments, the time domain
data size was maintained at 32 K, and the number of scans was
256, 129 and 500, respectively, with a relaxation delay of 2 s
between each transient.
For the selectively detected 4Q-SQ correlation experiments,
the indirect dimension pertains to 4th quantum, and the direct
dimension corresponds to single quantum. The spectral widths
2 1
of 7.99 and 2.00 ppm were employed in F and F dimensions,
respectively. Eight scans for each FID and a recycle delay of 4 s
were employed. The optimized t delay was 10.6 ms. The 4th
quantum signal was selectively detected by the appropriate
gradient ratio between 4Q-SQ of 4 : 1. SAPbz1H was prepared as
[
16]
reported earlier.
Chemicals
Scheme 1. Activation of the –N=CH- in (a) in vivo and (b) in vitro
chemical reactions.
The chemicals used for the synthesis of N-heterocycles and metal
complexes were purchased from Merck company. The solvents
were distilled prior to their use.
Reaction products: bis(N–salicylaldiminato)nickel(II), Ni{o–C6H4
(O)CHNH} (1); bis(N–methylsalicyladiminato)nickel (II), Ni{o–C H
6 4
1
13
1
The H and C{ H} NMR spectra of the Schiff base and the
nickel complexes were recorded in DMSO-d /CDCl on a Bruker
2
(O) CHNMe}2 (2); and bis(N-ethylsalicyladiminato)nickel (II),
Ni{o–C H (O)CHNEt} 0.5 H O (3).
6
3
Avance DRX 500–NMR spectrometer equipped with 5 mm
inverse–detection probe and Z–gradient coil, at ambient temper-
ature with TMS as an internal reference. The operating frequen-
6
4
2
2
To an ethanolic solution (10 ml) of nickel chloride (0.142 g)/
bromide (0.131g)/acetate (0.149g)/perchlorate (0.154g) (0.60mmol)
was added SAPbz1H (I) (1.20 mmol; 0.375 g) dissolved in ethanol
(10 ml). To this mixture four drops of liquor ammonia (25 %) were
added, and the mixture was refluxed on a steam-bath for about
6 h, during which an orange coloured solid was obtained. The
solid was separated from the solution by filtration (the filtrate
was preserved for further workup to isolate 4–aminophenyl benz-
imidazole) and washed with ethanol and dried in vacuo (yield:
1
13
cies for H and C are 500.13 and 125.74 MHz, respectively. The
1
13
experimental parameters for one-dimensional H/ C NMR
spectra used were as follows: spectral width: 16/250 ppm, data
points: 16 K/16 K, spectral resolution: 0.18/0.28 Hz, number of
accumulations: 8/16 k, acquisition time: 0.8/0.25 s, relaxation
delay employed: 2/3 s, and pulse length: 9.2/7 ms.
The structural assignments were made by the extensive utiliza-
tion of two-dimensional (2D) COSY, TOCSY, H – C HSQC, HMBC
experiments using standard pulse sequences.
1
13
1.270 g; 70%). The solid (brownish yellow) was analysed for the
[
15]
0
For COSY, the
formula Ni(C
7
H
6
NO)
2
(1); mp > 250 C. Analytical data: calcd for
spectral widths of 3 ppm were used in both direct and indirect
C
14
H
12
N
2
O
2
Ni, C, 56.24; H, 4.01; N, 9.37; Ni, 19.64 %. Found:
dimensions, and the size of the 2D data matrix was 128 ꢁ 1024.
C, 56.58; H, 4.33; N, 9.57; Ni, 20.28 %. meff, 1.30 BM. Infrared data
(KBr, cm ): 3300, 1533, 1618, 1330.
-
1
Four transients were acquired for each of the 128 t
1
increments
with a relaxation delay of 1.5 s. In the case of TOCSY, the spectral
widths of 7 ppm were used in both incremented and detected
dimensions, and the acquisition data size was 256 ꢁ 1024. Eight
A similar reaction in the presence of methylamine in place of
ammonia produced a clear solution. The solution was reduced
to 5 ml by evaporation under reduced pressure and was allowed
to stabilize at room temperature for overnight. A green coloured
solid obtained was washed with ethanol and dried in vacuo. The
1
transients were accumulated for each of 256 t increments with
1
.5 s relaxation delay. For HSQC experiment, the spectral width
of 3 ppm in the F
2
dimension and 80 ppm in the F
1
dimension
solid (pale green) was analysed for the formula Ni(C
8
H
8
NO)
2
(2);
ꢀ
were used. The optimized t delay for efficient transfer of magne-
tization was set to one bond JCH of 145 Hz. The size of the time
domain data was 128 ꢁ 512 points, and the number of accumula-
mp 202 C. Analytical data: calcd for C16
H
16
N
2
O
2
Ni, C, 58.77; H,
4.89; N, 8.57; Ni, 17.96 %. Found: C, 58.72; H, 5.16; N, 8.40; Ni,
-
1
17.89 %. m , 1.05 BM. Infrared data (KBr, cm ): 3055–3030,
eff
tions was four for each t data point with a relaxation delay of
1543, 1624, 1336.
1
1
8
7
.5 s between transients. The gradient ratio was maintained at
0 : 20. In the case of HMBC experiment, spectral widths of
ppm in F dimension and 80 ppm in F dimension were used.
When ethylamine was used in place of ammonia, the pale
green coloured solid analysed for the composition Ni (C H NO)
9
10
ꢀ
0.5 H O (3); mp 170 C. Analytical data: calcd for C H N O2.5Ni,
2
1
2
2
18 21 2
The delay responsible for polarization transfer was optimized
C, 59.39; H, 5.77; N, 7.69; Ni, 16.13 %. Found: C, 59.44; H, 5.80;
1
3
1
-1
for C– H coupling of 8 Hz. The size of the 2D data was
28 ꢁ 2048 points. The number of scans per each increment
N, 7.86; Ni, 15.73 %. m , 1.57 BM. Infrared data (KBr, cm ):
eff
1
2951–2862, 1541, 1610, 1330.
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Copyright © 2012 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2012, 50, 335–340

Activation of – N=CH – bond in a Schiff base by divalent nickel monitored by NMR evidence
Isolation of 4–aminophenyl benzimidazole (4–APbz1H, Ia): The
preserved filtrate (obtained from the reaction mixture, discussed
Results and Discussion
above) was treated with saturated aqueous sodium chloride solu-
tion (15 ml) followed by ether (15 ml). Subsequently after thor-
oughly shaking the mixture, the non-aqueous ether layer was
separated and dried over anhydrous sodium sulfate. The dried
ether layer was filtered, and ether was removed from the filtrate
by evaporation under reduced pressure. A pale yellow solid
obtained was dried in vacuo. The solid was analysed for the
Nickel(II) salts in ethanol react with SAPBz1H (I ) in molar ratio
(l : 2) and ammonia or alkyl amine at refluxing temperature to
produce 4-APbzlH (Ia) and complexes of the compositions
Ni{o–C
6
H
4
2 6 4 2 6 4
(O)CHNH} (1), Ni{o–C H (O)CHNMe} (2), Ni{o– C H
(O)CHNEt}
2
ꢂ 0.5H O (3) (for the structures please see Scheme 1)
2
The complexes 1, 2 and 3 have been structurally characterized
earlier using X-ray diffraction studies. Complex 1 obtained as an
orange red diamagnetic square planar has Ni-N and Ni-O bond
ꢀ
formula C13
for C13
H, 5.38; N, 20.17 %. Infrared data (KBr, cm ): 3439, 3362, 1620, 1600.
H N
11 3
(Ia; Scheme 1); mp 240 C. Analytical data: calcd
[
17]
H N
11 3
, C, 74.64; H, 5.26; N, 20.09 %. Found: C, 73.67;
lengths of 1.84 Å.
Complex 2 isolated as an a-isomer has
-1
exhibited phase transition from orthorhombic to monoclinic in
1
13
a
Table 1. H and C NMR spectral data of SAPbzlH, 4-APbzlH and metal complexes
a
a
Proton assignments
d, ppm
Carbon
d, ppm
assignments
SAPbzlH
4-APbzlH
SAPbzlH
4-APbzlH
NH
4
12.95 s
7.53 d
12.40s
7.48br
(7.9)
b
2
4
150.7 s
151.0 s
b
b
b
(7.4)
111.2br
122.5br
121.7br
114.0 d (160.0)
121.7 d (160.0)
121.7 d (160.0)
114.0 d (160.0)
138.3 s
7
7.68 t
7.46 s
(7.9)
5
(8.4)
6
5
2
3
.6
7.20q
6.7, 8.6)
7.60 d
8.3)
7.11 m
(ꢃ)
7
116.6 d (161.0)
143.9 s
(
(
(
8
’,6’
’,5’
6.70 d
(8.6)
9
135.0 s
138.3 s
1’
149.1 s
152.1 s
8.25 d
8.3)
7.84 d
(8.6)
2’,6’
4’
122.0 d (162.0)
128.6 s
113.5 d (157.99)
115.8 s
OH
12.97 s
-
-
3’,5’
127.5 d (162.0)
128.0 d (158.0)
NH
2
5.60s
SAPbzlH
Proton assignments
Complex
1
2
3
RN = CH
RN = CH
9.06 s
H
7.74 d
9.04 s
10.02 s
(
11.8)
R =
H
CH
3
3 2
CH , CH
†
8
.60 d (7.8)
3.48 s
1.40 t, 4.26s (6.7),
3”
4”
5”
6”
7.00 t
6.64 d
(8.4)
7.07 d
(7.0)
7.11 d
(8.1, 8.8)
(7.40)
7.44 t
7.13 t
(6.8)
7.15 t
(7.0)
7.23 m
(8.9)
(
7.3)
7.00 t
6.50 t
(7.0)
6.41 d
(6.8)
6.37 t
(8.1, 8.8)
(7.3)
7.68 t
8.4)
7.31 d
(7.8)
6.60 d
(7.8)
6.48 d
(
(8.4)
Carbon assignments
SAPbzlH
Complex
1
2
3
1
2
3
4
5
6
”
”
”
”
”
”
119.2 s
120.6 s
121.3 s
161.1 s
121.2 s
160.3 s
162.8 s
158.8 s
118.8 d (162.0)
133.5 d (159.0)
120.2 d (159.0)
133.4 d (150.9)
114.4 d (162.3)
133.3 d (157.0)
164.5 d (169.0)
124.3 d (152.2)
133.3 d (155.6)
115.2 d (159.8)
131.7 d (158.4)
164.7 d (158.6)
125.8 d (162.0)
132.9 d (167.1)
115.5 d (167.1)
131.3 d (167.1)
163.2 d (157.9)
b
113.5br
132.6 d (160.0)
163.8 d (165.0)
CH
C
c
4
6.9q (141.2)
18.9q (127.8)
d
5
3.8 t (140.0)
a
Spectra have been recorded in dmso-d
6
, CDCl
3
; coupling constants in Hz are given in parentheses; s = singlet, d = doublet, t = triplet, m = multiplet,
b
br = broad,
c
3
CH ,
d
2
CH .
†
14
2
The CH signal is a singlet (instead of a quartet as expected) owing to N quadrupolar interaction.
Magn. Reson. Chem. 2012, 50, 335–340
Copyright © 2012 John Wiley & Sons, Ltd.
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M. Chandrakala et al.
[
25,26]
the temperature range of 298–40 K, with Ni-N and Ni-O bond
distances 1.89–1.93Å and 1.83–1.85 Å, respectively. Complex 3
sequences have been reported for spin topology filtering.
[
18]
In situations when the spin topologies of coupled networks are
entirely different and in the absence of any interaction between
the coupled networks, the spectral complexity has been
obtained as yellow solid has Ni-N and Ni-O bond lengths of
[
19]
1
.92 and 1.84 Å, respectively,
and this has also been studied
[
20]
[27–29]
by UV–vis spectroscopy.
However, the complexes have not
unraveled by the higher quantum correlation experiments.
been subjected to NMR spectral investigations before.
In the present study the protons of different phenyl groups
of the investigated molecule are separated by more than five
bonds and do not exhibit any detectable scalar couplings
among them. The hydroxyl proton of a phenyl group also
did not display any coupling to the remaining protons. Thus,
The IR spectrum of 4-APbzlH exhibited bands at 3439, 3362,
-1
1
NH,
602 and 1600 cm assignable to nNH2, n nC=C and nN=C,
respectively. The mass spectrum of 4–APbzlH showed a molecu-
lar ion peak at m/z of 210 corresponding to M + 1 species.
1
Complex 1 is partially soluble in CHCl
3
but completely soluble
the signals of H nuclei at each phenyl ring can be regarded
in DMF and DMSO. Complexes 2 and 3 are soluble in common
organic solvents, and the solutions are non-conducting.
The IR spectrum of complex 1 showed a prominent sharp
as an isolated spin system of four coupled protons. The non-
selective excitation of 4th quantum (4Q) utilizing the hard
pulse pertains to the simultaneous flipping of all coupled
protons, giving rise to a single resonance at the cumulative
additive value of their chemical shifts. If there is a sizeable
difference in the algebraic sum of their chemical shifts, the
4th quantum single quantum (SQ) correlation spectrum yields
three different peaks in the indirect dimension of 2D spec-
trum corresponding to three different coupled networks. The
cross section taken parallel to the single quantum dimension
at any of the transition in the 4Q dimension pertains to
the single quantum spectrum corresponding to each phenyl
ring. Thus, the utilization of higher quantum detection not
only results in spin system filtering but also significantly
simplifies the spectrum aiding in the unambiguous assignment
of peaks.
-1
absorption peak at 3300 cm assignable to nNH of the nitrogen
[
21]
coordinated to the metal ion.
Such a peak is absent in the IR
[
22]
spectra of complexes 2 and 3. All the three complexes exhib-
ited an absorption peak below 1572 cm , and this is assigned
-1
to nNH of the coordinated ligand moiety. The nCO of the phenolic
-1
group observed at 1276 cm , in the spectrum of SAPbz1H has
-1
shifted to around 1335 cm in the spectra of complexes 1 to 3,
suggesting the linkage of oxygen to the metal ion.
The solid state magnetic moments of nickel(II) complexes at
ambient temperature are in the range of 1.0–1.6 BM (see Experi-
mental). The values are nearer to low spin square planar environ-
[
23]
ment rather than tetrahedral around nickel(II).
The electronic spectra of complexes 1 to 3 in DMF exhibited
-1
1
absorption bands in the range of 16 000–20 000 cm . The bands
are assigned to d ! d transitions arising from square planar
nickel(II) environment. Complex 2 displayed an additional band
The H 4Q-SQ correlated spectrum of SAPbzlH (Fig. 2) recorded
using the reported pulse sequence distinctly differentiates the
overlapped sub-spectra of the three coupled groups of protons
3”, 5”; 4”, 6” and 4 to 7. The 4Q chemical shifts pertaining to each
cross section are marked in the figure. In the direct dimension,
the multiplicity pattern based on the spin topology of the cou-
pled protons enabled the assignment of peaks of each chemically
-1
[24]
at 9775 cm , suggesting a tetrahedral environment for Ni(II).
These observations imply that there could be square planar and
tetrahedral equilibrium in solution.
The ESI mass spectra of complexes 1 to 3 exhibit a large num-
ber of peaks, some of them being higher than those of molecular
ion peaks. The fragmentation patterns are reported in Scheme SI
different proton. The F cross section taken at 4Q chemical shift
2
of d = 31.60 ppm in F dimension resembles the spectrum of an
1
/
/
(Supporting Information).
AA BB type spin system and is assigned to the central phenyl
ring. Depending on the multiplicity patterns expected based on
the spin topologies, the cross sections taken at d = 29.61 and
The solid state room temperature ESR spectrum of complex 2
using DPPH as the reference did not reveal the presence of
unpaired electrons implying square planar environment for nickel(II).
29.12 ppm in the F dimension are respectively assigned to phe-
1
” ”
nyl rings with protons numbered 4 to 7 and 3 to 6 . In each
group of the coupled spin system, the assignment to different
NMR spectroscopy
1
The H NMR spectrum of SAPBzlH (I, in DMSO–d
6
) displayed
two signals at d = 12.97 and 12.95 ppm and were assigned, re-
spectively, to the protons of OH and NH of phenolic and
imidazole moieties. A signal at d = 9.06 ppm was assigned to H
of N = CH. The spectral assignments for the Schiff base have been
[16]
reported earlier
and are further substantiated by routine two
dimensional COSY, TOCSY, HSQC, HMBC and multiple quantum-
single quantum correlation experiments in the present study.
All data are collected in Table 1.
1
It is evident from the H NMR spectrum (F
2
projection of Fig. 1)
that there is a severe overlap of spectral transitions arising from
the protons of three phenyl rings, resonating over a small spec-
tral range. As an example, a triplet detected at d = 7.68 ppm
assigned to protons 7 and 6” has overlapped peaks. The spin
topology indicates that these protons must yield different
multiplet patterns, and their identification is difficult from the
one-dimensional spectrum. In circumventing such problems,
the strategy of spin system filtering has been employed for
unambiguous assignment of resonances. The appropriate pulse
1
1
Figure 1. 2D H- H TOCSY spectrum of SAPbzlH.
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Copyright © 2012 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2012, 50, 335–340

Activation of – N=CH – bond in a Schiff base by divalent nickel monitored by NMR evidence
protons has been made using both the multiplicity pattern and
chemical intuition. Thus, the 4Q-SQ spectrum confirmed the
unambiguous assignment of peaks. Complexes 1, 2 and 3 have
1
13
been studied using H and C NMR including 2D and DEPT
experiments. The spectra confirmed the presence of methyl and
ethyl groups in complexes 2 and 3, respectively. The assignments
of the resonances are given in Table 1, and detailed discussion is
made in the supplementary information.
Reaction mechanism
The reaction mechanism of activation of –HC=N- in the Schiff
2+
base SAPbzlH (I) in the presence of Ni and ammonia or N-alkyl
amine may be visualized as illustrated in Scheme 2.
2+
Nucleophilic attack of the phenolic oxygen of I on Ni leads to
the formation of Ni-O bond (II) and release of a proton [evidence
is drawn from the formation of bis(salicylaldehydato)nickel(II)
[
30]
dihydrate from nickel acetate and salicylaldehyde]. The proton
1
activates -HC=NR by electrophilic attack to produce carbonium
Figure 2. 4Q-SQ Spectrum of SAPbzlH.
ion (III). The latter ion is susceptible for nucleophilic attack by
ammonia or N-alkyl amine, RNH
C-N- bond (IV). Subsequently, the earlier –C-N-bond undergoes
activation and gets cleaved to produce 4-aminophenyl
2
and produces an additional
–
Scheme 2. Proposed reaction mechanism of –N=CH- activation.
Magn. Reson. Chem. 2012, 50, 335–340
Copyright © 2012 John Wiley & Sons, Ltd.
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M. Chandrakala et al.
[
9] S. Naiya, H. S. Wang, M. G. B. Drew, Y. Song, A. Ghosh, Dalton Trans.
benzimidazole (Ia). In the process the new –C-N- bond changes into
C=N- so as to form a reformed Schiff base (V) intact with the
metal ion. This undergoes cyclization with the metal ion via N-
2
011, 40, 2744 and references therein.
–
[
10] E. Szlyk, A. Wojtezak, E. Larsen, A. Surdykowski, J. Neumann, Inorg.
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1
of –HC=N-R of another SAPbzlH molecule coordinated to the
[
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2
Schiff base leads to four-coordinate neutral complex of Ni(II),
1
[
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Acknowledgements
The authors are thankful to the UGC, New Delhi, Govt. of India, for
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analyses and Molecular Biophysics Unit, Indian Institute of
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Prof. N. Suryaprakash, NMR Research Center, Indian Institute of
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